1. Field of the Invention
The present invention relates to a process for producing a polyurethane grinding tool and, in particular, to a process for producing a porous grinding tool for grinding a copper-plated roll, the grinding tool being suitable for grinding the surface of a copper-plated roll for gravure platemaking.
2. Description of the Related Art
Copper-plated rolls for gravure platemaking are made by subjecting a tubular or cylindrical iron core to copper plating, and grinding the surface thereof by means of a porous grinding tool so that the surface is mirror-finished to a degree such that gravure platemaking and printing can be carried out satisfactorily. That is, as shown in FIG. 1, a copper-plated roll 1 for gravure platemaking is rotated at a predetermined speed, and at the same time a porous grinding tool 2 is rotated and run (preferably back and forth) along the axial direction of the roll 1 in intimate contact with the surface of the roll 1 so as to mirror-polish the surface of the roll 1 with the grinding tool 2.
In this case, surface grinding of the roll 1 generally employs a wet grinding method, in which the load imposed on the porous grinding tool 2 is appropriately adjusted and the positions of the grinding tool 2 and the roll 1 are adjusted so that the grinding tool 2 is in contact with the surface of the roll 1 with an appropriate contact area and pressure, and the grinding tool 2 and the roll 1 are rotated around their respective axes of rotation at suitable speeds and the grinding tool 2 is moved back and forth along the axial direction of the roll 1 while water is sprayed onto a grinding site via a nozzle (not illustrated) provided to one side. The angle of inclination of the grinding tool relative to the axis of the roll that is to be ground (angle of the grinding tool) is usually set at less than 0.5°, but in order to increase the grinding speed, there are cases in which the angle is set at greater than 0.5°.
As shown in FIG. 1 or FIG. 2, the grinding tool 2 is typically formed in a disc or squat cylinder shape, and usually has a diameter of about 200 mm and a thickness of about 50 to about 100 mm. The grinding tool 2 is provided with a through hole 3 in its central portion, which communicates coaxially with a hollow rotating shaft 4 for the suction and discharge of grinding debris.
The present applicant has previously disclosed a polyvinyl acetal porous grinding tool as a grinding tool for grinding a copper-plated roll for gravure platemaking (JP-A-2000-24935 (JP-A denotes a Japanese unexamined patent application publication)).
This polyvinyl acetal porous grinding tool is formed by mixing or impregnating a polyvinyl acetal resin, which is a matrix, with a thermosetting resin and an acrylic resin for the purpose of imparting water resistance thereto, but there is still a tendency for the grinding tool to absorb water that is used during grinding and swell, and the pore portion of the grinding tool decreases, thus causing to a certain extent the defect that the grinding tool is easily clogged. In addition, the polyvinyl acetal porous grinding tool disclosed in JP-A-2000-24935 has the drawback that when the angle of the grinding tool during grinding is increased, grinding scratches are easily caused, probably due to the low elasticity of the grinding tool itself.